Various types of optical reflection densitometers have been used in the prior art to measure the optical density of ink printed on paper or other stock by printing presses. One type, illustrated by U.S. Pat. No. 3,748,046 to James E. Murray, measures light reflected from the paper and also measures reference light not reflected from the paper, for example reference light relected from a reference area built into the densitometer; the light measurements are combined to produce indications of ink density.
A second type of prior densitometer measures only light which is reflected from the paper. A densitometer of the second type is described in U.S. Pat. No. 3,756,725, to J. M. Manring, in which a densitometer head is placed close to a rotating cylinder of a printing press, and a flash lamp in the densitometer head illuminates both a printed test patch area and an adjacent unprinted reference area on paper on which images and test patches were printed by the press. In U.S. Pat. No. 3,756,725, the flash lamp was used in common for simultaneous measurement of the reflectivity of both the printed test patch area and the unprinted reference area, and separate sensing apparatuses were employed for measuring the reflected light from the printed and unprinted areas respectively. Measurements made by the two measurement apparatuses for areas of the same optical density can be made equal by occasional manual adjustment of a gain setting device in at least one of the two measurement apparatuses of U.S. Pat. No. 3,756,725. The gain adjustment device can be manually adjusted to produce a density reading of zero when equal reflectivity surfaces are measured simultaneously by the two measurement apparatuses. The resulting gain setting produces equal overall measurement sensitivity for the test patch and the reference area measurement apparatuses. Unprinted paper is ordinarily put into the press to make the gain adjustment.
The present invention relates to the second type of densitometer, and in particular to compensating its density readings automatically to account for differences in measurement sensitivities of its two measurement apparatuses without adjusting their gains to have equal sensitivities.